1. Field of the Invention
The invention relates generally to an electrophysiological (“EP”) catheter system and method for providing energy to biological tissue within a biological site, and more particularly, to an EP system and method for assessing the adequacy of contact between the catheter and tissue and controlling the delivery of RF energy to the tissue based on the flow of fluid through the biological site.
2. Description of the Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (“SA node”) located in the wall of the right atrium. The SA node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (“AV node”) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth, remodeling, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the SA and AV nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as “cardiac arrhythmia.”
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed percutaneously, a procedure in which a catheter is introduced into the patient through an artery or vein and directed to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (“AF”), a procedure published by Cox et al. and known as the “Maze procedure” involves the formation of continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. In this therapy, transmural ablation lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. In this sense transmural is meant to include lesions that pass through the atrial wall or ventricle wall from the interior surface (endocardium) to the exterior surface (epicardium).
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In both the unipolar and the bipolar methods, the current traveling between the electrodes of the catheter and between the electrodes and the backplate enters the tissue and induces a temperature rise in the tissue resulting in ablation.
During ablation, RF energy is applied to the electrodes to raise the temperature of the target tissue to a lethal, non-viable state. In general, the lethal temperature boundary between viable and non-viable tissue is between approximately 45° C. to 55° C. and more specifically, approximately 48° C. Tissue heated to a temperature above 48° C. for several seconds becomes permanently non-viable and defines the ablation volume. Tissue adjacent to the electrodes delivering RF energy is heated by resistive heating which is conducted radially outward from the electrode-tissue interface. The goal is to elevate the tissue temperature, which is generally at 37° C., fairly uniformly to an ablation temperature above 48° C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100° C. In clinical applications, the target temperature is set below 70° C. to avoid coagulum formation. Lesion size has been demonstrated to be proportional to temperature.
In order to produce effective transmural lesions it is necessary to ensure that the electrodes are in intimate contact with the tissue. Positioning of the electrodes is typically done visually under fluoroscopy imaging and is thus largely a function of a physician's training and experience. Assessment of adequate electrode/tissue contact is somewhat of an art and verification, at present, is typically inferred through comparison of pre- and post-ablation electrocardiogram (“ECG”) analysis.
The use of impedance as an indication of electrode/tissue contact has been reported in the treatment of focal arrhythmias, such as ventricular tachyarrhythmia. In these procedures, a catheter with a single combination ablation/impedance-measuring tip electrode is inserted into the local blood pool within the heart and an impedance measurement is taken. The tip electrode is then placed at an ablation location and, so as to push the tip electrode deep into the cardiac tissue, force is applied along the axis of the catheter. An impedance measurement is then taken and compared to the impedance of the blood pool. This subsequent impedance measurement is referred to as a “contact-assessment” impedance. A significant increase in the contact-assessment impedance relative the blood-pool impedance serves as an indication that the tip electrode is in contact with cardiac tissue.
In this procedure a significant increase in impedance is noted due to the fact that the tip electrode is pushed deep into the cardiac tissue and is thus largely surrounded by tissue, as opposed to blood. While this electrode/tissue contact assessment technique is effective for the treatment of focal arrhythmias, it is less effective for the treatment of non-focal arrhythmias, such as atrial fibrillation. Ablation therapy for atrial fibrillation often involves the formation of transmural linear lesions. In this form of ablation therapy a linear array of band electrodes is placed against the atrial wall. While the band electrodes are held against the tissue with some degree of force, a portion of the band electrodes is likely to remain in the blood pool. The presence of blood against a portion of the band electrode affects the impedance measurement and reduces the significance of the difference between the blood-pool impedance and the contact-assessment impedance. Thus, the above-described electrode/tissue contact assessment technique that relies on the use of a tip electrode forced into the tissue is ineffective for linear ablation therapy. This known technique is further ineffective for linear ablation because it does not account for fluctuations in impedance measurements which may occur due to movement of electrodes caused by respiration and heart contractions.
Blood coagulation is a major limitation/complication associated with RF ablation therapy. Coagulation can lead to thromboembolism and also form an insulating layer around the electrode hindering further energy delivery required for ablation therapy. Thus, heating of blood is a major concern for ablation safety and efficacy. During ablation therapy, it is known that the temperature of blood near an electrode is dependent on the blood flow rate. Low blood flow results in reduced convective heat dissipation within the blood pool around the electrode and thus higher blood temperature. Conversely, high blood flow rate results in increased convective heat dissipation within the blood pool around the electrode and thus a lower blood temperature.
Conventional RF ablation systems fail to account for the effect that varying blood flow rates have on blood, electrode and tissue temperatures, which can be substantial. During an ablation procedure, conventional systems apply a level of RF energy to the electrodes sufficient to elevate the tissue temperature to a level that causes the tissue to become nonviable. The level of RF energy is generally constant regardless of the blood flow rate and is only adjusted if the system employs some type of temperature feedback control. In these systems an attempt is made to guard against blood coagulation and coagulum formation by monitoring the temperature of the electrodes, usually using a thermocouple attached to the electrode. When a threshold temperature is reached, the application of RF energy is either reduced or shut off. However, such thermocouples are generally located at the tissue/electrode contact location, which can have a significantly different temperature than the opposite side of the electrode that is in the blood pool.
Such systems tend either to have a high incidence of coagulation or to operate inefficiently. Coagulation is likely to occur in these systems when the RF energy delivered to the electrode is set to an ablation-inducing level during periods of high-blood flow. The temperature sensing thermocouple does not provide the system with sufficient information about the temperature of the blood pool. Consequently, the convective heat dissipation effect of the high-blood flow keeps the blood pool around the electrode cool and ablation is efficiently accomplished, however, during periods of low-blood flow, the reduced convective heat dissipation allows the blood pool to heat. Over the course of an ablation procedure, the cumulative effect of the periods of low-blood flow is likely to result in coagulum formation.
In order to avoid coagulation the energy level may be reduced. This, however, tends to lead to an inefficient ablation procedure. If the energy level is set to induce ablation during periods of low flow, the convective heat dissipation effect during periods of high-blood flow reduces the electrode temperature and thus the tissue temperature to a non-ablative level. The culmination of these periods of non-ablative temperature levels at best increases the amount of time necessary to achieve an ablation-inducing temperature and thus the overall procedure time, and at worst prevents the electrode from ever reaching an ablation-inducing level.
Hence, those skilled in the art have recognized a need for a RF ablation system and method that assesses the adequacy of electrode-tissue contact independent of impedance measurements and controls and adjusts the RF energy level delivered to tissue within a biological site based on the flow rate of fluid through the site. The invention fulfills this need and others.